In the past, image pickup apparatuses have used an electronic correction function for correcting shaking (hand-shaking) when the image pickup apparatus is held in a person's hand. Such an image pickup apparatuses control a shutter speed (exposure time) and an analog gain. The analog gain indicates to what extent a signal of an image captured at the shutter speed should be amplified. Accordingly, a preview image and a shake-corrected still image can have the same brightness. To reduce image shake, when a user presses a shutter-button, a terminal captures a still image at a shutter speed faster than a shutter speed calculated by an auto-exposure unit at the time of preview. In this manner, a still image is captured quickly reducing motion from the shake captured in the image, and a shake-corrected still image can be obtained.
Cutting the shutter time decreases image brightness, so in order to prevent a difference in brightness between the preview image and a shake-corrected still image, analog gain is amplified. The analog gain is amplified more than during the preview of the image, to amplify the analog image signal, and thus maintain image brightness at the same level. However, amplification of the analog gain causes random noise, thereby degrading the image quality. The random noise can be removed by capturing a plurality of still images and combining the captured still images.
The above-described countermeasures against random noise have been used; however, few countermeasures have been taken against flicker. When an image is taken using an image pickup apparatus when all or part of the illumination for the subject of the image is from a periodic light source, the image may have periodic bright and dark patterns across the image. A fluorescent lamp is the most common example of a source of periodic light. A ballast (i.e., an electrical power control device) of a fluorescent lamp can produce periodic frequencies in the intensity of light output of the fluorescent lamp. The periodic frequencies of the fluorescent illumination can create aliasing effects with the image scanning synchronization frequency of the image pickup apparatus. This effect may be amplified when a plurality of still images are taken at fixed time intervals (i.e., periodically) to correct for random noise.
Thus, when flicker appears in a captured still image, even if random noise is removed from the image, the image may still be unclear. Thus, there is a need for reducing flicker appearing in an image captured using an electronic hand-shake correction function.